Wonder if the process of learning to drive in those countries has an effect. I recall hearing that the US system is pretty lax when it comes to what's required to get a license, and that a fair few places basically have you drive around a parking lot/a few blocks rather than any sort of formal test.
On the other hand, while it's not super challenging, the UK one at least has a decent amount of work you need to put in to get a license, with a system involving two tests and examiners actually willing to fail people (even for things that wouldn't really be all that dangerous in practice).
It sounds like you're being rhetorical, but that sounds totally reasonable to me. It seems believable that one country might have some variables that cause drivers to be more distracted by phones, and when the safety margins of attention are so small, even a small difference in average distractedness could equate to 50% more deaths.
The key issue is that our roads are designed to maximize vehicle speed, and this kills people.
Focusing on the responsibilities of drivers ignores the environment that they operate in, which is clearly the worst in the developed world.
Americans will demand big roads, big cars and long distances, all at taxpayer expense, and then try to blame some other group when these things kill people.
Shrink roads, shrink cars, reduce amounts driven, or continue killing ~6000 Americans per year for your personal convenience